Create Both

MaildirStore local
  Path @maildir@/
  Inbox @maildir@/INBOX
  Subfolders Verbatim

IMAPAccount fastmail
  Host imap.fastmail.com
  User alyssa@fastmail.com
  PassCmd "cat ~/imappass"
  SSLType IMAPS
  SSLVersions TLSv1.2

IMAPStore fastmail-remote
  Account fastmail

Channel fastmail
  Master :fastmail-remote:
  Slave :local:
  Patterns *
  SyncState *